Union Pacific ordered 9 Stainless Steel RPO's from Budd in 1963 complete with the mail bag hook attachment on the door....... but I don't think they made it to Amtrak 8 years later in May '71.
http://rrpicturearchives.net/pictures%5C56011%5C286a%20UP%205908.jpg
Six of the cars went to VIA and...